The period of July 16-20, 1999 included the most sustained severe pollution event during the 1999 Northeast Oxidant and Particulate Study (NEOPS) summer campaign. Concentrations of O3 exceeding both the 1-hour and 8-hour National Ambient Air Quality Standards (NAAQS) were widespread both within the Philadelphia metropolitan area and across the mid-Atlantic region. Concentrations of fine particles (PM2.5) were also enhanced. This paper provides the meteorological background for understanding the air quality observations made during this episode as part of the NEOPS campaign. Meteorological features consistent with prior sustained pollution episodes in this region are presented as well as several unique features that impacted observations at the NEOPS site in north Philadelphia. The onset phase of this episode followed the “standard” form for regional scale O3 events in the mid-Atlantic. An upper level ridge developed west of the region with local subsidence and westerly transport aloft. PM2.5 and O3 concentrations rose quickly. However, an unusual period of stagnation, driven by a low-level cyclonic circulation embedded within the broad upper level ridge, marked the mature phase of the episode. While O3 concentrations gradually rose throughout the episode, peak PM2.5 concentrations (in excess of 60 mg/m-3) did not occur until the termination phase of the episode when esterly transport was re-established. Similarities and differences with other multi-day pollution episodes during the NEOPS campaign will be discussed with respect to observed transport regimes.